Protein adsorption by very dense porous zirconium oxide particles in expanded beds

ABSTRACT

Porous Zirconia particles of specific gravity of 2.5-3.5 g/cm 3  and mean particle sizes of 30-400 μm can be synthesized using oil emulsion methods from colloids and used for protein adsorption in stable expanded beds. Expanded beds of less than 1.0 settled bed height to diameter (approximately 10 ml bed volume) are stable at linear fluid velocities of at least about 100 cm/hour.

STATEMENT REGARDING GOVERNMENT RIGHTS

The present invention was made with government support from the NationalScience Foundation, under Grant No. CHE-9107029 (June 1991), and fromthe National Institutes of Health, under Grant No. 5R01-GM45988 (June1990). The Government has certain rights in this invention.

This is a continuation of application Ser. No. 08/394,714, filed Feb.27, 1995, now U.S. Pat. No. 5,837,826, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The potential advantages of expanded beds, i.e., fluidized beds, of ionexchange or affinity adsorbents for direct adsorption of proteins frombiological process liquids and particulate-containing fluids has beendemonstrated. Conventional fluidized bed adsorption systems use 100 μmto 400 μm polymeric or composite particles with small densitydifferences between the adsorbent particles and the liquids beingprocessed. The wide particle size distribution of these adsorbentsresults in stable beds of particles being classified by the fluidvelocity and density. Even though these classified expanded beds arestable, many of these particles are large and have a long characteristicdiffusion length (i.e., path length from the outer surface of theparticle to the center) within the adsorbent. This results in pooradsorption kinetics. Furthermore, such adsorption particles lack robustligands with high specificity, are unable to be repeatedly cleaned withharsh reagents, and cannot be used for protein adsorption at elevatedtemperatures (i.e., greater than about 40° C.). In addition, loss of bedcapacity with increasing bed expansion (liquid flow rate) usingconventional fluidized bed adsorbents may limit fluidized bedseparations due to pore mass transfer resistance.

Solutions to these and other problems associated with conventionalfluidized bed systems have included the use of magnetically stabilizedfluidized beds (MSFB) and the division of the bed into stages. A moreuseful approach would be to develop stable adsorbent particles of higherdensity with appropriate adsorption properties. A need exists for suchparticles. It is envisioned that with denser particles, higherfluidization velocities (100-250 cm/hour) can be achieved with smalleradsorbent particles. While high flow rates can also be achieved usinglarger particles (200 μm to 500 μm), fluidization of small particleswill minimize bed dispersion and result in more rapid protein adsorptionin the presence of entrained particulates such as fermentation broths,cell lysates, blood, or cell culture fluids.

Porous ceramic particles, such as silica, as well as highly crosslinkedfunctionalized organic polymeric materials are used in high performanceliquid chromatography for the separation of proteins. However, most ofthese particles do not have the appropriate density for effective use inexpanded or fluidized beds. Furthermore, the ability to repeatedlyremove adsorbed protein, nucleic acids, lipids, pyrogeniclipopolysaccharides (LPS), and intact virus or microorganisms (bacteria,fungi or yeast) from such chromatographic media is a challengingproblem. This makes these materials undesirable for use in fluidized bedapplications because process scale protein adsorbents useful forpurification of therapeutic or diagnostic proteins must be capable ofrepeated clean-in-place cycles. Both the adsorbent and the surface needto be stable to cleaning without loss of capacity or mechanicalstability. Such cleaning methods are generally harsh: high or low pHsolutions (0.1-2 M NaOH, formic, acetic, peracetic, trifluoroacetic, orhydrochloric acid to 1 M) often in alcohol (70% ethanol or 30%isopropanol); high ionic strength solutions (2 M NaCl or KCl); non-ionicdetergents; or high temperature conditions followed by extensive washingwith purified, sterilized buffer. Particularly vigorous methods, oftenat elevated temperatures (e.g., 40-80° C.), are needed to remove andinactivate lipopolysaccharide endotoxin (LPS) and viral nucleic acidsfrom protein adsorbent media because of the extremely low residuallevels of these contaminants allowed in human biologics as establishedby federal regulation and the World Health Organization. Somesilica-coated and organic polymeric chromatographic adsorbents toleratecleaning with 0.1 M sodium hydroxide or ethanol-acetic acid mixtures atrefrigerated (4° C.) or ambient temperatures and are more mechanicallystable than carbohydrate adsorbents. In general, however, organicpolymeric adsorbents cannot be cleaned with harsh agents at elevatedtemperatures (e.g., 40-60° C.) or sterilized with steam or direct heat.Furthermore, silica chromatographic adsorbents typically cannot berigorously sanitized, for example, with 0.2 M to 1 M sodium hydroxide,without degradation. In general, silica-based materials are not stableoutside the pH range of 2 to 8.

Small (<25 μm) surface-modified porous and highly dense zirconium oxideparticles are used in high performance liquid chromatography separationsof proteins. See, for example, J. A. Blackwell et al., J. Chromatogr.,549, 59 (1991); J. A. Blackwell et al., J. Chromatogr., 596, 27 (1992);P. W. Carr et al., Chromatography in Biotechnology, American ChemicalSociety, Washington D.C., Symp. Ser. 529, 146 (1993); J. Nawrocki etal., J. Chromatogr. A, 657, 229 (1993); and J. Nawrocki et al.,Biotechnol. Prog., 10, 561 (1994). Such surface-modified zirconium oxideparticles can be modified with a variety of Lewis bases. Only certain ofthese surface-modified zirconium oxide particles, however, are capableof withstanding the repeated harsh cleaning conditions required forprocess scale separation of polypeptides and proteins destined fortherapeutic use. Furthermore, such particles do not have the appropriateparticle size for effective use in an expanded fluidized bed.

Although larger zirconium oxide particles (1 μm to 1 cm), have beensuggested as suitable for use in fluidized beds (see, U.S. Pat. Nos.5,108,597, 5,271,833, and 5,346,619), these particles are eithercarbon-clad or carbon-clad with a crosslinked polymer. Suchsurface-modified zirconium oxide particles are used for reversed phaseseparations and are not generally suitable for expanded bed and mostprocess scale protein separations. See, for example, C. H. Lochmuller etal., Preparative Chromatography 1, 93 (1988). Furthermore, proteinadsorption at elevated temperatures and repeated cleaning of theseparticles with strong base (0.2 M to 1.5 M NaOH) has not been disclosed.Thus, a need exists for adsorbent particles of the appropriate particlesize, density, porosity, and high temperature stability for use inprocess scale expanded beds for purification of therapeutic ordiagnostic proteins.

SUMMARY OF THE INVENTION

The present invention provides a system and method of separating atarget protein from a feedstock in an expanded bed. This method includesthe steps of: expanding a bed of surface-modified zirconium oxideparticles, wherein the surface-modified zirconium oxide particles have acapacity factor greater than about 10 (preferably greater than about 20and more preferably greater than about 50) and comprise a core zirconiumoxide particle having a particle size of about 30-400 μm (preferably50-200 μm) and a specific gravity of about 2.5-3.5 g/cm³ (preferablyabout 3.0-3.5 g/cm³); eluting the feedstock through the expanded bed toadsorb the target protein to the surface-modified zirconium oxideparticles; and removing the target protein from the surface-modifiedzirconium oxide particles. Preferably, the surface-modified zirconiumoxide particles comprise an ion-exchange phase, such as a Lewis base,examples of which include fluoride, phosphate, citrate, maleate, EDTA,EGTA, CDTA, borate, polyphosphate, dicarboxylic acid, and tricarboxylicacid; or an affinity phase such as hydrophilic polymer, such as apolyamino acid, or a carbohydrate polymer having covalently boundaffinity ligands, examples of which include a carbohydrate polymer, suchas dextran, having covalently bound triazine dyes, thiophilic ligands,or other nonprotein affinity ligands.

Advantageously, the step of eluting the feedstock through the expandedbed to adsorb the target protein to the surface-modified zirconium oxideparticles is preferably carried out at a linear fluid velocity of atleast about 100 cm/hour, and the step of removing the target proteinfrom the surface-modified zirconium oxide particles is preferablycarried out without reversing the flow of the eluent. Significantly, thebinding capacity of the expanded bed of surface-modified zirconium oxideparticles at 1% breakthrough is preferably at least about 20 mgprotein/ml settled bed volume and the terminal settling velocity ispreferably about 2-4 mm/second in water at ambient temperatures.

The method of the present invention is advantageous because thefeedstock containing the target protein can include entrained solids,e.g., cells or cellular debris, such as bacteria, yeast or blood cells.If desired, the step of eluting the feedstock through the expanded bedis carried out at a temperature greater than about 30° C., preferablygreater than about 50° C., which is particularly advantageous for veryviscous feedstocks that flow at elevated temperatures.

Advantageously, the method can further include a step of cleaning thesurface-modified zirconium oxide particles with a strong base, e.g., 0.2M NaOH, which can be repeated with little or no deterioration of theparticles. For certain surface modifications, a strong base will stripthe particles of the surface modification. If this occurs, the surfacemodified particles can be regenerated by contacting the cleanedparticles with a surface-modifying material, e.g., NaF.

The present invention also provides a method for producing inorganicparticles using a series of steps referred to herein as the surfactantoil emulsion (SOM) method. This method includes the steps of: dispersingan aqueous sol comprising a colloidal dispersion of inorganic particleswith a forming/extracting medium comprising a nonionic surfactant;heating the dispersion of the aqueous sol and forming/extracting mediumto extract water from the sol and form aggregates of sol particles;collecting the aggregates; washing the aggregates; and sintering theaggregates at a temperature and for a time effective to increase theirmechanical strength. Preferably, the forming/extracting medium comprisespeanut oil, and more preferably a mixture of peanut oil and oleylalcohol. Preferably, the step of heating comprises heating at atemperature of about 80-100° C.

The present invention also provides a method for producing inorganicparticles using a series of steps referred to herein as the fed batchoil emulsion (FBOM) method. This method includes the steps of:dispersing an aqueous sol comprising a colloidal dispersion of inorganicparticles in a forming medium using an in-line mixer to form anemulsion; combining the emulsion with an extracting medium to extractwater from the sol and form aggregates of sol particles; collecting theaggregates; washing the aggregates; and sintering the aggregates at atemperature and for a time effective to increase their mechanicalstrength. In this FBOM method, as opposed to the SOM method, the formingand extracting media are used to separate the emulsion step from thedrying step. In the FBOM method, although they can be, the forming andextracting media are not the same media Also, provided are the sinteredporous ZrO₂ particles prepared by the FBOM and SOM methods.

The following abbreviations are used throughout: AFS₂₈₀ =full scaleabsorbance at 280_(nm) ; BSA=bovine serum albumin; C=bulk proteinconcentration; C_(o) =column inlet protein concentration; C_(i) =UVmonitor voltage; CDTA=cyclohexanediaminetetraacetic acid; (D_(ax))_(app)=apparent axial dispersion coefficient; DBC=dynamic binding capacity,C/C_(o;) ε=bed voidage; EDTA=ethylenediaminetetraacetic acid;EGTA=ethylene glycol-bis(β-aminoethyl ether)N,N,N',N'-tetraacetic acid;FBOM=fed batch oil emulsion; H=bed height; LPS=lipopolysaccharide; m_(o)=tracer peak zeroth moment; MES=2-morpholinoethanesulfonic acidmonohydrate; MSFB=magnetically stabilized fluidized bed; OEM=oilemulsion synthesis; Q=volumetric flow rate in tracer studies;RTD=residence time distribution; SOM=surfactant oil emulsion synthesis;Δt=tracer peak data acquisition time interval; u=superficial liquidvelocity; and u_(t) =particle terminal settling velocity. As usedherein, an expanded bed and a fluidized bed are used interchangably,without limitation as to particle size distribution of the adsorbent inthe bed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B. 45° cone flow distribution for ascending fluidization ofporous zirconia particles.

A. Stainless steel screen held in place with sealing ring.

B. Flat screen held in place with teflon ring.

FIG. 2. Schematic of the Fed Batch Oil Emulsion (FBOM) process.

FIGS. 3A-3B. SEM of (A) SOM particles and (B) FBOM particles(magnification 100×).

FIG. 4. Bed expansion as a function of linear velocity. Combined SOMparticles, 4 mg/ml BSA in MES loading buffer.

FIGS. 5A-5B. Tracer residence time distributions as a function of flowdistributor geometry with and without particles in the column.

A. Flow system without particles, 45° inlet flow adaptor; flow rate 9.5ml/minute; nitrate tracer.

B. Flow system with particles, 45° inlet flow adaptor; 1× and 2×, flowrate 9.5 ml/minute; 2.5× flow rate 13.5 ml/minute; 3× flow rate 18.8ml/minute; nitrate tracer.

FIGS. 6A-6D. Adsorption breakthrough for BSA by fluoride adsorbedcombined SOM particles as a function of bed expansion. Symbols representtriplicate determinations. Settled bed height 2.3 cm.

A. Packed bed; C_(o) =4.2 mg/ml; fluid velocity=109 cm/hour.

B. 2× expansion; C_(o) =3.9-4.0 mg/ml; fluid velocity=110 cm/hour.

C. 2.5× expansion. C_(o) =4.0-4.1 mg/ml. Fluid velocity=158 cm/hour.

D. 3× expansion. C_(o) =4 mg/ml. Fluid velocity=200 cm/hour.

FIGS. 7A-7C. BSA adsorption/elution cycle using fluoride adsorbed SOM Aparticles (50 μm) and a 2.5 cm bed diameter. Settled bed height 2.1 cm.The full scale absorbance (AFS₂₈₀) for washing, equilibration, andloading=0.5. AFS₂₈₀ for elution=2.0, except for (C) where AFS₂₈₀ forwashing and loading=1.0.

A. Packed bed; C_(o) =4.0 mg/ml; fluid velocity=109 cm/hour; load=50 mMMES, 100 mM NaF; elution=50 mM MES, 100 mM NaF, 750 mM Na₂ SO₄ ;wash=0.1 M NaOH.

B. 2× bed expansion; C_(o) =4.0 mg/ml; fluid velocity=107 cm/hour;load=50 mM MES, 100 mM NaF; elution=50 mM MES, 100 mM NaF, 750 mM Na₂SO₄ ; wash=0.1 M NaOH.

C. 3× bed expansion; C_(o) =4.0 mg/ml; fluid velocity=215 cm/hour;load=50 mM MES, 100 mM NaF; elution=50 mM MES, 100 mM NaF, 750 mM Na₂SO₄ ; wash=0.1 M NaOH.

FIG. 8. Dynamic BSA binding capacity of fluoride adsorbed SOM A & Bparticles as a function of bed expansion. Breakthrough calculatedfollowing the method of Chase and Draeger, J. Chromatogr., 597, 129(1992); •, C/C_(o) =1%; ▴, C/C_(o) =5%.

FIG. 9. Time required to increase flow rate to final fluidizationvelocity during BSA adsorption on fluoride adsorbed SOM A & B particles.C/C_(o) ≈4 mg/ml; •, settled bed and 2× expansion; ▴, 2.5× bedexpansion; ♦, 3× and 3.4× bed expansion.

FIGS. 10A-10B. The effect of flow disruption geometry and particleclumping on BSA binding capacity by fluoride adsorbed SOM A particles.

A. Comparison of stainless steel frit and 8 μm screen; bed expansion 2×;▪, stainless steel frit. DBC at C/C_(o) =0.05, 30±2 mg/ml settled bedvolume; •, screen (configuration A, FIG. 2). DBC (C/C_(o) =0.05) 31±2mg/ml settled bed volume.

B. The effect of particle clumping on DBC. Bed expansion 2×. •, cleanparticles, no clumping (screen configuration B, FIG. 2). DBC (C/C_(o)=0.05) 32±4 mg/ml settled bed vol. ▪, clumped particles. DBC (C/C_(o)=0.05) 9±2 mg/ml settled bed vol.

FIG. 11. Adsorption breakthrough for BSA by fluoride adsorbed SOMparticles at elevated temperatures (2× bed expansion).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an expanded bed process and system thatincludes an adsorbent material that is exceedingly mechanically stableat temperatures in excess of 40° C.; base-stable (i.e., such that it canwithstand repeated cleaning with base as described below); sufficientlydense and of an appropriate particle size for effective fluidizationresulting in stable bed expansion; and sufficiently porous forseparation of proteins from large volumes of fluids. This adsorbentmaterial possesses a significantly high and surprisingly substantiallyconsistent dynamic protein binding capacity when expanded at high fluidvelocity (i.e., greater than about 100 cm/hour). Thus, it can be used toadsorb proteins much more rapidly in expanded beds than eitherinorganic-composite or crosslinked carbohydrate polymeric particles.Furthermore, the expanded bed adsorbent material of the presentinvention can significantly reduce column cycle time (equilibration,loading to breakthrough, washing, protein elution, cleaning), and cost,in processing large volumes of protein-containing process fluids. It canalso be used advantageously for adsorbing proteins from a viscousmaterial that must be heated to flow, such as a concentrated sugarsyrup, and for adsorbing proteins from a solids-containing processstream such as cell homogenates, culture fluid, or milk.

In an expanded (or fluidized) bed, a sedimented bed of an adsorbent isexpanded by upward flow of a liquid fluidization medium, e.g., water,buffer, or other aqueous media, in a column (typically, a verticallyheld column with a solution inlet in its bottom portion and a solutionoutlet in its top portion) with a flow distribution system designed toreduce axial mixing. When the bed is stably expanded, as defined below,and has expanded at least greater than 1× times (typically up to 3×times) its original sedimented bed height, a feedstock is applied. Asthe feedstock passes through the expanded bed, the target protein, whichis either to be removed as a contaminant or collected as a desiredproduct, is adsorbed onto the adsorbent material and the remainingfeedstock material, which can be in the form of solids such as cells orcellular debris, passes through. In standard expanded beds, the flow isthen reversed to elute the target protein from the sedimented adsorbent.Although this can be done in the system and process of the presentinvention, the eluent flow does not need to be reversed. That is,advantageously the target protein can be eluted quickly and effectivelywithout changing the direction of flow, or the flow rate.

The parameters influencing the efficiency of the separation in anexpanded bed include the type of adsorbent material (e.g., size,density, and surface properties), the feedstock properties (e.g.,viscosity, biomass content, temperature, etc.), and the flow distributordesign. The expanded bed adsorbent material described herein produces astable bed, i.e., a bed of stably suspended particles. That is, there isgenerally no apparent internal circulation, i.e., no visible boiling onthe surface or internal jets. Furthermore, there are no detectableparticles or fines in the effluent and no detectable clumping of theparticles which would cause mixing and nonuniform liquid flow throughthe bed reducing protein binding capacity. This bed stability can bemaintained in extremely shallow beds (i.e., less than 1:1 height todiameter) with high flow rates, i.e., at least about 100 cm/hour, withlow pressure drops and without magnetic bed stabilization.

Furthermore, the expanded bed adsorbent material described herein has asignificantly high and surprisingly substantially constant dynamicprotein binding capacity even at high fluid velocities. That is, at 1%breakthrough, i.e., the point at which 1% protein appears in theeffluent, the binding capacity under flow conditions is at least about20 mg protein/ml settled bed volume, preferably at least about 25 mgprotein/ml settled bed volume. The "settled bed volume" is the volume ofa bed of particles settled by gravity in an aqueous media For use inrapid process scale separation, binding capacities need to be well inexcess of 5-10 mg protein/ml settled bed volume. Also, the dynamicprotein binding capacity of the expanded bed adsorbent materialdescribed herein increases with increasing temperature.

The expanded bed system described herein is significantly fast atprocessing proteins. That is, large amounts of protein can be loaded ona shallow bed (i.e., a bed of 1:1 height to diameter or less) in a shortperiod of time. For example, 250-400 mg of protein can be completelyadsorbed on 10 ml of adsorbent material of the present invention inunder 10 minutes.

The expanded bed adsorbent material of the present invention includesparticles having a zirconium oxide core, the surface of which ismodified for protein separation applications with a surface-modifyingmaterial. The preferred core zirconium oxide particles are generallyspherical and can be referred to as "spherules." Thus, the terms"particle" and "spherule" are used herein interchangeably. Althoughparticles made by the preferred process described below have very fewflattened portions or indentations, such deviations from perfectspherical particles are tolerated in expanded bed applications. Thus,the present invention is also intended to encompass irregularly shapedparticles. The core zirconium oxide particles are chemically andmechanically stable in an aqueous system having a wide pH range (i.e.,pH 1-14) and/or under extreme temperatures (e.g., autoclavingtemperatures and even temperatures as high as 500° C.). They aretypically strong enough to withstand an expanded bed system withsubstantially no decomposition or generation of fines. Such stabilityallows protein separation from viscous liquids that require heat toreadily flow through a fluidized bed.

The mean particle size of the core zirconium oxide particles is within arange of about 30-400 μm. Below this particle size, the particles becomeentrained in the eluent, and above this particle size, the particleswill not fluidize. Preferably, the mean particle size of the corezirconium oxide particles is about 40-300 μm, and more preferably about50-200 μm. As used herein, "particle size" is defined by the average ofthe longest dimension of each particle and can be measured by anyconventional technique. Preferably, this is the average "diameter" ofthe particles because preferred particles are generally spherical inshape. Thus, the terms "diameter size," and "particle size" are usedinterchangeably.

The core zirconium oxide particles have a specific gravity (i.e., theeffective particle density during fluidization or water-filled density)of about 2.5-3.5 g/cm³, preferably about 3.0-3.5 g/cm³. Neither theparticle size nor the specific gravity are changed significantly whenthe surface of the particles is modified for protein separations.Particles of this range of particle size and density can be fluidized athigh flow rates (i.e., at least about 100 cm/hour) and form a stable bedwithout magnetic bed stabilization.

The core zirconium oxide particles have generally uniform pores, thediameter of which depends on the size of the colloidal particles used intheir preparation and on the process for bringing the colloid togetherto form large pore spherules. The larger the colloidal particles, thelarger the pores between them in the spherules. Typically, the pores areless than about 0.15 μm (1500 Å) in diameter. Preferably, for proteinseparation applications, the particles have a pore size of about200-1000 Å, more preferably about 400-1000 Å, and most preferably about800-1000 Å. For preferred embodiment, the pore size distribution is alsogenerally relatively narrow. Preferably, greater than about 70% (andmore preferably greater than about 90%) of the pores have a pore sizewithin a range that spans ±50% of the average pore size. The voidfraction is preferably greater than about 0.4, and more preferablygreater than about 0.5.

Because bare zirconium oxide particles bind proteins irreversibly, thezirconium oxide particles used in expanded bed applications are modifiedto reversibly bind proteins. Modified particles having a high specificaffinity are suitable for expanded bed applications. Modified particleswith a high specific affinity have a large capacity factor, k', which isthe measure of the strength of adsorption. That is, k' is the fractionof the solute in the adsorbed phase over the fraction of solute in thesolution phase. For an expanded bed adsorbent material, the capacityfactor k' should be high in the loading phase and low (preferably lessthan 1, and ideally 0) in the washing phase. This can be adjusted bycontrolling the elution conditions, and is well known to one of skill inthe art.

Such modified particles, i.e., those with a high specific affinity,include those capable of adsorbing proteins using an ion-exchangemechanism or an affinity mechanism. Such particles are referred to ashaving an ion-exchange phase and an affinity phase, respectively. Suchmaterials have a capacity factor of greater than about 10, preferablygreater than about 20, and more preferably greater than about 50.Excluded from such materials are reversed-phase materials, e.g.,carbon-coated particles, and materials coated with hydrophobic polymers.Examples of suitable coating materials include carbohydrate polymers,such as dextran or cellulose, with covalently bound triazine dyes,thiophilic ligands, and other nonprotein affinity ligands, hydrophilicpolymers such as polyethyleneimine and polyamino acids, and hard Lewisbases such as fluoride, phosphate, maleate, citrate, EDTA, EGTA, CDTA,borate, polyphosphates, di- or tri-carboxylic acids. It should be notedthat ion-exchange coating materials are less desirable than affinitycoating materials for use in the direct recovery of many proteins fromfermentation broths or cell culture fluids, due to the high ionicstrength of these fluids and the resulting low adsorbent capacities.

These modified particles must be base-stable. As used herein,base-stable means that the particles must withstand repeated cleaningwith base (e.g., 0.2 M to 1.5 M NaOH) at ambient or elevatedtemperatures (e.g., up to about 100° C.) without significantdeterioration. This does not mean, however, that the material modifyingthe surface is not removed by base. It can be, as long as the surfacemodification is easily regenerated. For example, the significant basestability of fluoride-modified particles allows sterilization of thematerial and stripping of bound proteins with 0.2 M sodium hydroxidesolutions. Although such treatments strip all adsorbed materials,including fluoride, equilibration of the stripped particles with afluoride solution of the desired ionic strength and pH restores theselectivity and efficiency of the expanded bed adsorbent material to itsoriginal condition. In preferred embodiments, this cycle of sorption andstripping can be repeated indefinitely with substantially no loss ofefficiency or selectivity. Thus, although ion-exchange phases may beless desirable than affinity phases when used in expanded beds, thefluoride-modified zirconium oxide particles are a convenient, easilycleaned and regenerated adsorbent material.

The surface-modified porous zirconium oxide particles with theprescribed particle size and density described above are well suited forfluidization, even in beds with height to diameter ratios of slightlyless than 1.0. With conventional fluidized bed materials, e.g., organicpolymers or silica, a bed with a height to diameter ratio of at least3:1 is required. Using the surface-modified zirconium oxide spherulesdescribed herein, stable expanded beds of classified particles over arange of linear fluid velocities at least about 100 cm/hour, and as highas 4000 cm/hour, can be attained. Typically, for 50 μm particles, alinear fluid velocity of about 100-400 cm/hour for a 2× expansion ispossible, preferably it is about 100-200 cm/hour. For 150 μm particles,a linear fluid velocity of 700 cm/hour for a 2× expansion, and 1750 fora 3× expansion, are possible. For 200 μm particles, a linear fluidvelocity of 1350 cm/hour for a 2× expansion, and 3100 for a 3×expansion, are possible. Thus, using the adsorbent material describedherein, the speed at which proteins can be adsorbed and feedstocks canbe processed provides a significant advantage.

The terminal settling velocity, u_(t) (i.e., the maximum velocity atwhich the particles will settle under the influence of gravity), ofuseful particles is about 2-4 mm/second, preferably about 2.7-3.3mm/second, in water at ambient temperatures (i.e., 25-30° C.). This isconsiderably faster that the terminal settling velocity of commerciallyavailable 100-300 μm DEAE particles (available under the tradenameSTREAMLINE™ from Pharmacia, Inc., Uppsala, Sweden) of 0.7 mm/second. Theminimum velocity at which the particles are fluidized will be slightlyhigher than the terminal settling velocity. A particle with a lowterminal settling velocity will be fluidized at a low velocity and hencethe throughput to the system will be low (the rate at which protein canbe adsorbed).

Although the adsorbent material described above is particularlyadvantageous for fluidized bed applications, the stability of theexpanded beds and the ability to clean the particles can be affected bythe flow distribution geometry. Thus, a unique inlet flow distributionsystem was created to enhance the effectiveness of the modifiedzirconium oxide particles by reducing the back-mixing caused by radialvelocity differences during fluidization in the column. This systemincludes an inlet flow distribution system as shown in FIG. 1B. FIG. 1Ais shown for comparison purposes as this arrangement resulted inturbulent mixing. In FIG. 1B, an inlet screen 1, such as a ceramic,stainless steel, or other base-resistent screen, is attached to aconical- or dish-shaped end fitting 3 with a sealing ring 5 inside acolumn 7, such as a glass or stainless steel column. The inlet screen 1is attached inside the column 7 with the sealing ring 5 in a manner thatreduces turbulence, e.g., back-eddies, above the sealing ring 5. Thedesign shown in the exploded view of FIG. 1A, wherein the screen is bentand the top of the sealing ring is at the level of the screen, orslightly below, causes such turbulence. Thus, the design shown in FIG.1B is preferred. The top of the conical- or dish-shaped end fitting 3 ispreferably of the same diameter as the internal diameter of the column7, although it can be slightly less, and the angle 9 is at least about45°, although it can be greater.

The present invention also provides methods for the preparation ofporous zirconium oxide particles. For purposes of the present invention,an adequate method is one that yields generally mechanically andchemically stable porous particles. Various colloid-aggregationprocesses can be used for the preparation of the porous zirconium oxidespheres described above. Preferred methods are based on an oil emulsiontechnique, which involves mechanically dispersing micron-scale dropletsof an aqueous zirconia sol of colloidal particles in an oil phase.Gelation of the colloids within the droplet and extraction of water fromthe droplets yields zirconia aggregates that are further strengthened bysintering. The larger the colloidal granules used in the formation ofthe aqueous sol, the larger the resulting pore diameters. These methodsproduce a polydisperse collection of spheres. Such particles arepreferably produced in a yield of greater than about 30%, morepreferably greater than about 50%, and most preferably greater thanabout 70%. Yields greater than about 90% are possible with the fed batchoil emulsion process described below.

In one oil emulsion method, referred to herein as the surfactant oilemulsion ("SOM") method, the extraction and gelation occur as theemulsion is being continuously formed. A nonionic surfactant is used tohelp control the final particle size. Specifically, the method involvesdispersing an aqueous sol containing a dispersion of colloidal zirconia(ZrO₂) particles into droplets distributed in a forming and extractingmedium that will extract water from the dispersed sol and form droplets.This forming/extracting medium includes a substantially nonpolar liquidor mixture of liquids that will form an emulsion with the polar sol andthat also has sufficient polar character to extract water from the soldroplets. The colloidal sol contains a nonionic surfactant to stabilizethe droplet sizes that will result in particles of the desired size.Optionally, the forming/extracting medium can include a base or the solcan include a water-soluble base precursor, e.g., urea, to inducegelation of the sol droplets before they dry. All or a portion of thewater is removed with heating while stirring at high speeds to reformdroplets as they coalesce. The drying results in gelled, solid spherulesthat consist of aggregated colloid particles that can be convenientlyseparated from the medium by any suitable method, e.g., filtration.

The forming/extracting medium for the SOM method includes asubstantially water-insoluble, high molecular weight oil, a long-chain,substantially oil-miscible alcohol or ketone, or mixture thereof. Theoil, alcohol, and ketone can have substantially carbon-based backbonesor silicon-based backbones. Preferably, the SOM forming/extractingmedium includes a mixture of an oil and an alcohol or a ketone. The oilcan be any oil with a sufficiently high molecular weight such that ithas a very low volatility but is reasonably fluid. Examples of suitableoils include vegetable oils, such as peanut oil, corn oil, olive oil, aswell as silicone oils, and hydrocarbon oils. The alcohol or ketone canbe any alcohol or ketone having greater than 7 carbons that issufficiently miscible with oil and sufficiently immiscible with water toform an emulsion with water. Preferably, the alcohol or ketone has a lowvolatility such that it does not boil off to any significant extentduring the forming and extracting processing steps. Also, it isdesirable if it does not air oxidize to any significant extent. Ifdesired, the alcohol or ketone can be substituted with a sulfoxide, forexample, or any compound that has at least one hydrophilic end and thatsuppresses the surface tension of the water. A preferredforming/extracting medium contains peanut oil and oleyl alcohol, whichare combined in a volumetric ratio of about 1:1 and used at atemperature of about 80-100° C. Peanut oil has a high viscosity andnonpolarity while oleyl alcohol adds extractive properties to themedium. Mixtures of the two allow viscosity, nonpolarity, and extractioncapacity of the medium to be controlled. Depending upon the ratio of solto forming medium, extraction times of about 1 minute to 4 hours can beused to fully gel the zirconia spherules.

In the fed batch oil emulsion process ("FBOM"), the emulsion iscompletely formed prior to extraction and gelation and the aqueouszirconia sol is concentrated. Concentration of the aqueous zirconia solbefore it is used to form an emulsion increases the stability of theemulsion. This eliminates the need for continuously reforming theemulsion with high speed rotary mixers as extraction and gelation occur.It also allows for an increase in batch size without a scaled increasein the amount of associated water that needs to be extracted. A nonionicsurfactant can be added to the sol to help control the final particlesizes as with the SOM method. Emulsion formation occurs by mixing thesol and a forming medium in an in-line mixer. The forming medium is anonpolar liquid or mixture of liquids that will form an emulsion withthe polar sol. Optionally, it may have some polar character as long asit is still capable of forming a two phase emulsion with the sol. Theemulsion is transferred to an extraction medium where the emulsion ismaintained while all or a portion of the water is removed with heatingand stirring. The extraction medium is a liquid with sufficient polarcharacter to extract water from the sol droplets and sufficient nonpolarcharacter to maintain the two phase emulsion created with the in-linemixer. Often, the extracting medium is also used as the forming medium,although the two steps are separate in the FBOM process. The dryingresults in gelled, solid spherules that consist of aggregated colloidparticles that can be conveniently separated from the medium by anysuitable method, e.g., filtration.

The FBOM forming medium can contain an oil, alcohol, ketone, or any ofthe materials described above for the SOM forming/extracting medium.Preferably, the FBOM forming medium contains at least an oil, alcohol,ketone, or mixture thereof, with carbon or silicon backbones havingabout 12-24 units. Such preferred materials have the appropriateviscosity, nonpolarity, and chemical stability for the oil emulsionprocess. Examples include vegetable oils such as peanut oil, saffloweroil, corn oil, canola oil, sunflower oil, as well as mineral oils,silicone oils, and hydrocarbon oils such as 12-24 unit linear andbranched alkanes. The FBOM extracting medium can contain an alcohol,ketone, or sulfoxide, or any of the materials described above for theSOM forming/extracting medium. Preferably, the FBOM extracting mediumcontains at least an alcohol, ketone, sulfoxide, or mixture thereof,with carbon backbones having about 8-24 carbons. These liquids have theappropriate nonpolarity, viscosity, nonvolatility, chemical stability,and water extraction capacities to maintain the two phase emulsion whileextracting water. Examples include, but are not limited to, octanol,decanol, octyl ketone, decyl ketone, octyl sulfoxide, and oleyl alcohol.The forming and extracting media can be the same liquid or mixture ofliquids, e.g., peanut oil and oleyl alcohol, although this is not arequirement.

Referring to the schematic in FIG. 2, the FBOM process can be describedas follows. A forming medium 3 and a concentrated sol 2 (layered withadditional forming medium 1 on top) are each pumped with peristalticpumps 4 and 5 to a "Y" 6 where the streams are combined. The combinedstream then passes through an in-line mixer 8 where the emulsion 9 isformed. The in-line mixer is shown with a turn 7 in it. Although this isnot required, it is preferred to enhance mixing of the concentrated sol2 and the forming medium 3. From the in-line mixer 8, the emulsion 9 issent to the batch vessel 10. The batch vessel 10 is filled with anextracting medium 12, which is heated with the heating bath 13 and mixedwith a batch mixing impeller 15, which is attached to motor 16. Theheating bath 13 can be filled with water or a higher temperature heatingfluid 17. It is preferably insulated with a porous material 18 toachieve consistent heat transfer to the batch vessel 10 and extractingmedium 12. After all the concentrated sol 2 has been transferred to thebatch vessel 10, the pumping is continued to pump the forming medium 1through the lines to clear the in-line mixer of the concentrated sol 2.Mixing and heating in the batch vessel is continued to eliminate waterfrom the droplets. When sufficient water has been eliminated, thedroplets form solid spherules consisting of aggregated colloid.Stability is tested by simulating the wash protocol on small samplesfrom the batch and observing them under a light microscope. Thespherules are collected and washed after they are judged to besufficiently stable.

Processing parameters, such as sol composition and pH, colloid size,medium composition, temperature of medium, and time of drying, forexample, are necessary to control for consistent particle formationusing either the SOM process or the FBOM process. For example, rotarymixing rates for the SOM process, and flow rates through the in-linemixer for the FBOM process control the sizes of the final particles.Although the particles used in the fluidized bed systems described aboveinclude a zirconium oxide core, the methods of the present invention canbe adapted to use other inorganic colloidal particles. That is, althoughthe expanded bed adsorbent material described above includessurface-modified zirconium oxide particles, the methods described abovecan be used to make other ceramic particles.

Thus, the inorganic colloidal particles used in the oil emulsionprocesses, i.e., SOM or FBOM, can be made of metals, metal oxides,metalloid oxides, as well as metal or metalloid oxide precursors (suchas hydroxides that can be converted to oxides at elevated temperatures)having one or more of their dimensions within a range of about 1 nm toabout 1 μm. These colloidal particles must also preferably be capable offorming a stable sol. As used herein, a "stable-sol" is asolid-in-liquid two phase system wherein the solid consists of colloidalparticles that do not begin to settle out of, or separate from, theliquid upon standing for about 2 hours (by visual inspection).Typically, this occurs if the inorganic particles are generally waterinsoluble and generally acid insoluble at a pH of about 2. Preferably,the particles have surface hydroxyl groups that assist in dispersing thecolloidal particles.

Suitable sources of inorganic oxides are also refractory, i.e., they donot melt or otherwise decompose (other than to convert to an oxide if inthe form of an oxide precursor, such as a hydroxide) at elevatedtemperatures. Preferably, they do not melt or otherwise decompose attemperatures up to about 500° C., and more preferably up to about 1000°C. Typically, suitable refractory particles have melting points greaterthan 1000° C. Lower melting oxides, or hydroxides that are converted tooxides by heating, can be used if the organic polymer constituent of theaggregate is removed by slow oxidation of the organic polymer at lowertemperatures.

By definition, colloidal particles have at least one of their dimensionswithin a range of about 1 nm to about 1 μm. Preferably, no dimension islarger than about 1 μm. Particles having any dimension larger than about1 μm are generally not suitable for use in preparing the desiredspherical particles of the present invention. Typically, irregularlyshaped, nonuniform, large particles form from such colloidal particles.More preferably, no dimension of the colloidal particles is larger thanabout 0.5 μm (5000 Å). Most preferably, the colloidal particles aregenerally spherical having an average particle size of about 30-150 nm(300-1500 Å).

The size of the colloidal particles contribute to the final pore size ofthe resultant sintered particles. Thus, it is particularly desirable forthe samples of colloidal inorganic particles used to be generallyuniform in size. By this, it is meant that at least about 70% of thecolloidal particles in a sample are within a range that spans ±50% ofthe average particle size of the colloidal particles.

Preferably, the inorganic colloidal particles include the metals inGroups IIIB, IVB, VB, and VIB, the metals and metalloids in Groups IIIA,IVA, and VA of the Periodic Table, as well as the lanthanides. In thiscontext, metalloids and oxides or hydroxides of metalloids such assilicon, aluminum, and germanium, as well as metals and oxides orhydroxides of metals such as zirconium and tin, are included within thescope of the term "inorganic particle" as long as the particles arecolloidal and can form a stable sol in water, preferably acidic water.If the inorganic particles, e.g., the oxides or the hydroxides thatconvert to oxides, are somewhat soluble in acid, they can be coated withan impervious layer of a less soluble inorganic oxide, such as silica,for example. Of these preferred inorganic particles, inorganic oxideparticles, e.g., metal oxides and metalloid oxides, are the morepreferred. Most preferably, the colloidal inorganic particles arealumina (Al₂ O₃), titania (TiO₂), as well as silica (SiO₂) and zirconia(ZrO₂) or both (ZrSiO₄). Again, although these colloidal particles canbe used in the oil emulsion methods of the present invention, onlyzirconium-based colloids are used in the preparation of particles foruse in fluidized bed systems and methods for the separation of proteins.

In particularly preferred applications, the colloidal particles arezirconium oxide (ZrO₂). Colloidal dispersions of zirconium oxidesuitable for use as the ZrO₂ source used to prepare the sinteredparticles of the present invention are manufactured by Nyacol Inc.,Ashland, Mass. These dispersions contain about 20 wt-% ZrO₂, wherein thecolloidal ZrO₂ particles vary in average diameter, e.g., from about10-250 nm. For example, Nyacol Zr 100/20 is an aqueous dispersioncontaining 20 wt-% colloidal ZrO₂ particles, the majority of which areabout 100 nm in diameter.

Minor amounts of noncolloidal sources of the desired inorganic particlescan be included within the sols used in the methods of the presentinvention. For example, noncolloidal ZrO₂ sources, i.e., those that donot produce a stable sol as defined above, can be included along withthe colloidal ZrO₂ particles used to prepare the spherules of thepresent invention. Thus, chloride, nitrate, sulphate, acetate, formateor other inorganic or organic salts of zirconium, such as the oxysaltsand alkoxides, can be included with the ZrO₂ sol and the mixture used tomake the final sintered polymer-free aggregates. In such mixtures,however, colloidal ZrO₂ particles make up a major part of the total ZrO₂present.

The final sintered particles that contain a metal oxide, e.g., ZrO₂, canalso include a minor amount (preferably less than about 20 mole-%) of asecondary metal oxide. For example, other metal oxides (or precursorsthereof) can be included in the sols of the desired metal oxide (orprecursor thereof), e.g., ZrO₂, so as to stabilize a particularcrystalline phase of the desired metal oxide or to retard grain growthin the sintered particles. For example, salts or oxides sols of metalssuch as yttrium, magnesium, calcium, cerium, aluminum, and the like, canbe included at levels of up to about 20 mole-% in a sol of ZrO₂. ZrO₂particles fired in air or in oxidizing atmospheres that do not containother oxide additives display either monoclinic, tetragonal, orpseudocubic crystal structures when cooled to room temperature. Higherfiring temperatures and longer firing times favor the presence of themonoclinic phase. The inclusion of other metal oxides allows thepreparation of particles that possess either monoclinic, tetragonal, orcubic crystalline structures.

The aqueous sol of dispersed inorganic oxide colloidal particles caninclude a miscible organic solvent capable of lowering the polarity ofthe liquid in the sol or capable of increasing the volatility of thedispersing phase of colloidal sol to increase drying and gelation ratesin the oil emulsion process. Organic solvents having a lower dielectricconstant than water can be used as long as they are miscible with waterand form a stable sol as defined above. The organic solvent should alsobe generally noninterfering in the gelling and extracting steps of theoil emulsion process. Suitable such miscible organic solvents includealcohols such as methanol, ethanol, propanol, isopropanol (i.e.,2-propanol), as well as acetonitrile, tetrahydrofuran, dioxane, anddimethylsulfoxide.

Finally, the inorganic particles are sintered to increase theirmechanical strength. Sintering may be visualized as the closing of poreswithin the aggregates due to the surface tension of the solid surfacewhich exerts a contracting force on the colloidal particles surroundinga pore. Since the compressive strength of the aggregates is low at hightemperature, the colloidal particles are drawn together and the poresbetween them shrink. A temperature that reduces the specific surfacearea of the aggregates by at least about 10% below the value obtained ona powder dried from the original sol is generally sufficient.Preferably, sintering occurs at a temperature of about 750-1000° C.,although temperatures above 1000° C. can be used. Sintering is carriedout for a time effective to increase the mechanical strength of theparticles a desired amount. Typically, about 1-15 hours is preferred.Sintering can be carried out in an oxygen atmosphere, although this isnot necessary.

Objects and advantages of this invention are further illustrated by thefollowing examples. The particular materials and amounts thereof recitedin these examples as well as other conditions and details, should not beconstrued to unduly limit this invention. All materials are commerciallyavailable except where stated or otherwise made apparent. Allpercentages are by weight unless otherwise specified.

Experimental Examples

I. METHODS

A. Synthesis of Porous Zirconium Oxide Particles Using the SOM Process

Porous zirconia particles were synthesized using an oil emulsion processin the presence of surfactant (SOM). One thousand angstrom zirconiumoxide colloid (Nyacol Lot IV-40) was centrifuged for two hours at1,590×g (3,000 rpm, Beckman JA 10 rotor) to select the larger granules.Batches of 80 g of colloid were resuspended by shaking in nitric acid,pH 3, to obtain a colloidal sol of approximately 16% (wt/wt). Onethousand two hundred ml of peanut oil (Baker's Secret) and 1,200 ml ofoleyl alcohol Eastern Chemical) were poured together to form the formingextracting medium, agitated using two 3-blade propellers (45° pitch) at450 rpm (lower propeller 10.5 cm diameter; upper propeller 7.6 cmdiameter; bottom propeller 2 cm above the bottom of the beaker; toppropeller 5 cm above bottom propeller) and concurrently preheated to 85°C. in a 18.3 cm diameter 4-liter polypropylene beaker in a boiling waterbath. Forty grams of urea and 0.33 ml of non-ionic surfactant (TRITON™X-100, available from Sigma Chemical, St. Louis, Mo.) per 80 g ofcolloid were dissolved in the resuspended sol. The mixture was thenpoured into the agitated, preheated oil and alcohol mixture withconstant stirring to create the sol/oil emulsion. The mixture was heatedand stirred for 4 hours in order to eliminate water from the soldroplets so that they would form solid aggregates. During the dryingprocess, the droplets densified, the colloid granules within thedroplets began to flocculate, and ultimately, the droplets solidifiedinto stable, intact particles containing totally aggregated colloid withlittle water. During drying, particle size and stability were monitoredusing light microscopy (230× magnification). When coalescence was nolonger observed and when split particles began to be observed, theagitation rate was reduced to 360 rpm. After 4 hours, drying wascomplete and the bath was removed from the heat. The particles weresettled for 5 minutes before the oil/alcohol mixture was decanted toremove the smallest particles. The remaining particles were washed 3times with 75-100 ml of hexane, three times with 25-50 ml of isopropylalcohol, and dried using vacuum filtration, after which the particleswere free-flowing.

Particles were sintered using a programmed temperature oven (NEY) for 2hours at 375° C., 6 hours at 750° C., and 3 hours at 900° C. A 40°C./minute temperature ramp was used to reach each temperature.

After cooling, free-flowing particles were acid and base treated toestablish a consistent surface chemistry. The particles were washedfirst with carbonate-free, double-distilled water with sonication(Bransonic® Ultrasonic Cleaner Model 1200) under vacuum for 15 minutes.The liquid was decanted and the particles washed and gently rocked inexcess 0.5 M carbonate-free NaOH on a shaker table overnight. Thesupernatant was decanted, the particles rinsed with carbonate-free,double-distilled water and the supernatant decanted. The particles werethen washed with gentle rocking in excess 0.5 M carbonate-free nitricacid on a shaker overnight, the supernatant decanted, and rinsed withcopious amounts of carbonate-free, double-distilled H₂ O and dried undervacuum at 100° C. for 8 hours. The particles were then sonicated incarbonate-free, double-distilled H₂ O under vacuum and stored incarbonate-free, double-distilled H₂ O until classification.

Particles were classified to achieve an average particle size of 50 μmby settling prior to elutriation (i.e., classification by flowingliquid). Settling was carried out in double-distilled H₂ O in agraduated cylinder (1 liter) followed by decanting the fumes from thetop. Elutriation was done with ascending fluidization either in a 30cm×1 cm (inside diameter, "id") (Ace Glass Co.) or a 15 cm×2.5 cm id(Kontes) glass column at a flow rate sufficient for 3- to 4-foldexpansion of the original settled bed volume for at least 30 minutes.Column height was adjustable with a hydraulic plunger positioned atleast 1 centimeter above the expanded bed height. During elutriation,fines left the column and the top of the bed was not well defined.Following elutriation, particles were settled overnight and then the bedwas progressively expanded. Any remaining small particles eluted until astable expansion was achieved for at least 10 minutes after 2-, 2.5-,and 3-fold expansion from the settled bed height. A stable bed expansionis defined as one in which there is no visible bed "boiling" wherein thesurface is disrupted, no fluid jetting wherein there are no streams ofhigh velocity liquid, and no fines being eluted from the top of the bed.Bed height was determined using a transparent scale attached to theoutside of the column. Particles were later removed from the columnfollowing BSA adsorption experiments and air dried for determination ofparticle size distribution.

Particles were characterized by determination of mean diameter andparticle size distribution by both screening (Small Parts, Inc., MiamiLakes, Fla.) and electronic particle size distribution (Coulter LS100)methods. Particle surface area and pore volume were determined bynitrogen BET adsorption and desorption using a PSI porosimeter(Micrometrics ASAP 2000 V3.00), as described by S. Brunauer et al., J.Am. Chem. Soc., 60, 309 (1938), and S. J. Gregg et al., AdsorptionSurface and Porosity, Academic Press, New York (1982). Particle specificgravity (i.e., the effective particle density during fluidization) wascalculated from measurements of apparent density and from nitrogenporisimetry data. Apparent density was determined by packing dryparticles into a 5.00 ml volumetric flask by successive filling andtapping until the particles were completely packed. The effectivedensity was calculated from the apparent density by assuming a voidfraction of 0.36 in the volumetric flask. Effective particle density wasalso calculated from nitrogen porisimetry data by assuming that thevolume of N₂ adsorbed/g ZrO₂ =particle void volume/g ZrO₂. Bothcalculations were based on a specific gravity of 5.7±0.1 g/cm³ formonoclinic ZrO₂. These methods for particle density determination werecompared with the observed particle density during fluidization.Particle morphology was examined using a scanning electron microscope(SEM, Hitachi 5-450). Viewing samples were prepared using Au/Pdsputtering at 15 μA in argon at 50 μm Hg pressure for 3 minutes.

B. Synthesis of Porous Zirconium Oxide Particles Using the FBOM Process

The Fed Batch Oil Emulsion (FBOM) process consisted of the followingfive steps. In the first step, a zirconia/nitric acid sol (Nyacol LotIV-40, 20%, 1000 Å, pH 3) was concentrated to 44.5% by centrifugationand resuspension. The sol was centrifuged for two hours at 1,590×g(3,000 rpm, Beckman JA 10 rotor) and 0° C. The supernatant was collectedand divided into six 500-ml centrifuge bottles and centrifuged for twohours at 17,700×g (10,000 rpm, Beckman JA10 rotor) and 0° C. The sixpellets were then resuspended in an amount of pH 3 nitric acid needed toend up with slightly more than 44.5% zirconia at the end of theconcentration process. This averaged about 38 g per centrifuge bottle orabout 228 g total. The resuspension was done using a 37° C. shakeroscillating at 240 rpm for 12 to 24 hours. A third centrifugation wasdone for 5 minutes at 17,700×g and 0° C. to pellet out aggregatedcolloid particles. The pellet was discarded. The density of the finalsupernatent was then measured to determine its weight percent zirconia.An amount of the original sol was added to bring its final concentrationdown to 44.5% zirconia.

Concentration was done for two reasons. First, it allowed the batch sizeto be increased greatly without an increase in drying time. For example,in the SOM process, 16% sol was used with 80 g of zirconia and 422 g ofnitric acid (water) present. At a concentration of 44.5%, if FBOM had422 g of nitric acid present, it would have 338 g of zirconia. So thebatch size could be increased 4.2 times without any increase in dryingtimes. Second, it allowed larger batches to be made with smaller totalbatch volumes (less oil phase) without adversely affecting theestablished droplet (particle) size distribution in the batch vessel.Normally, a higher sol-to-oil ratio would mean more collisions betweendroplets and increased rates of coalescence. But tests using theconcentrated sol under the conditions involved in the FBOM processresulted in no observable coalescence, while the unconcentrated sol wasobserved to readily coalesce under the same conditions. The eliminationof coalescence was thought to be due to the higher viscosity and surfacetension of the more concentrated sol. With coalescence virtuallyeliminated, not as much oil phase was needed to separate droplets. Thismeant that more concentrated batches of particles could be made withoutadversely affecting the desired particle size distributions.

For the second step, emulsion formation, 800 g of this finalconcentrated sol (444 g nitric acid, 356 g zirconia) was sent through anin-line mixer along with a forming medium. A two phase emulsion wascreated using 12 inches of 3/16 inch outside diameter (od) polyacetalin-line mixers (Cole-Parmer, Niles, Ill.) contained in 5/16 inch od(3/16 inch id) Tygon tubing (Cole-Parmer). A diagram of the apparatusused for this step and for the drying step is shown in FIG. 2. Theemulsion consisted of the previously concentrated zirconia colloid soland the forming medium consisted of peanut oil (Baker's Secret) andoleyl alcohol (Eastern Chemical) mixed in a 1:1 volume ratio. The solwas pumped at a rate of 37.5 ml/minute and the forming medium was pumpedat 111 ml/minute. The streams were combined using a glass "Y" (Kimax)and flowed through the in-line mixer at a rate of 148 ml/minute at a 1:3volume ratio (sol:forming medium). Pumping was accomplished usingperistaltic pumps. Temperatures of the sol and forming medium were about22° C. As the sol and forming medium were pumped through the in-linemixer, an emulsion was created consisting of sol droplets suspended inthe forming medium. The size distribution of the sol droplets was suchthat the final resulting particles would have a fairly narrow sizedistribution centered around particles with a mean diameter of 50 μm.

As seen in FIG. 2, a 180° turn was put in the in-line mixer to ensurethat the total volume passing through the in-line mixer underwent thesame shear stresses no matter where it entered the mixer (e.g., near thewall or near the center). With a turn such as this, there is a greaterradial exchange of material between the walls and the center of thein-line mixer. All of the sol was able to be completely pumped throughthe in-line mixer without air bubbles by adding 50 ml of the formingmedium to the top of the sol and pumping for an extra minute to clearthe lines of sol. In addition, during this minute, the sol line underthe glass "Y" was manually held up to allow the more dense sol to passthrough this low spot where it would otherwise pool. The total time foremulsion formation lasted about 13.5 minutes plus one minute to clearthe lines of sol. During this time, an emulsion with a total of 506.5 mlof sol (800 g) and 1647 ml of forming medium (1609.5 ml through the oillines, 37.5 ml through the sol lines) was pumped into the batch vesselwhere the third step, drying and particle formation, occurred.

The heated, uncovered, stirred batch vessel was used to maintain thedroplet size distribution while providing the conditions necessary todry the droplets and form solid, stable particles. This was accomplishedby maintaining the emulsion with agitation vigorous enough to keep thehigher density droplets suspended and to prevent droplet coalescence butmoderate enough to prevent droplet/particle breakage. This was carriedout in a 12-inch tall plastic batch vessel with dimensions of 7 incheswide by 10 inches long at its base diverging to 7.5 inches wide by 10.5inches long at its top. Before the addition of the emulsion, the vesselwas filled to 3.5 inches (3000 ml) with the extracting medium, which wasthe same as the forming medium (1:1 volume ratio of peanut oil to oleylalcohol). This was agitated with a centered, 5-inch diameter, 5-bladedimpeller (Jiffler brand available from Knox Lumber Co.) rotating at 295rpm under the power of an electric mixing motor (BDC 3030, Caframo,available from Baxter Diagnostics, Inc., Scientific Products Division,McGraw Park, Ill.) while it was preheated to 95° C. using a boiling hotwater bath insulated with foam.

When the temperature reached 95° C., the pumps were started to createthe emulsion. For the 14.5 minutes the emulsion flowed into the heated,agitated batch vessel, the batch temperature dropped to 76° C. from 95°C. and the batch depth increased to 5.15 inches. As the depth increased,the agitation rate was increased without the development of vortices,which cause air bubbles to be whipped into the batch. Vortices wereavoided because they can cause problems with particle to particleconsistency. The agitation rate was increased gradually to a final valueof 372 rpm from 295 rpm over a time period of about 7 to 8 minutes.

The batch emulsion was continually heated with the boiling hot waterbath and stirred at 372 rpm for a total of 90.5 minutes in order toeliminate the water from the sol droplets. The overall drying rateduring this time was equal to the mass of nitric acid at the start,minus the mass of nitric acid at the end, all divided by the totaldrying time. There were 444 g of nitric acid at the start. The dryingprocess took 90.5 minutes. Estimating from the final calculated voidfraction, there were (0.565 ml water/0.435 ml zirconia)(l mlzirconia/5.7 g zirconia)(356 g zirconia)=81 g (about 81 ml) of nitricacid at the end of the drying process. So the drying rate was about 4.0g/minute. Dividing by the amount of zirconia present, the drying rateper mass of zirconia was about 1.1 g water/g zirconia/minute. As thewater from the emulsion was eliminated, the sol droplets densified, thecolloid granules within the droplets began to flocculate, andultimately, the droplets solidified into stable, intact spherulescontaining totally aggregated colloid with little water present(estimate 356 g zirconia/(356 g+81 g water)=81% zirconia).

The status of the droplets/particles was monitored periodically duringthe drying process by examining samples under a 230× light microscope.Size, approximate density, coalescence, and stability of thedroplets/particles could be readily observed. In addition, stability wasalso tested by simulating the wash protocol on a small sample andexamining it under the microscope. The particles were considered stableto the solvent wash when the largest particles would survive the washand subsequent placement under a cover slip on a microscope slide. Theparticles were sufficiently stable to undergo collection and washing90.5 minutes after the start of emulsion formation.

When the droplets formed solid, stable particles, they were collectedand washed in the fourth step of the process using vacuum filtration andisopropanol. The particles were collected and washed in order toseparate them from themselves and from residual oil and water. This wasdone by the following procedure. After the particles were judgedsufficiently stable, the agitation was turned off and the batch wasallowed to sit for about 10 minutes. Even though almost all of theuseful particles (i.e., those with a particle size of at least 38 μm)settled out of the oil phase in this time, the smaller particles stillin the oil phase were collected in order to get a more accuraterepresentation of the size distribution of all the particles. The oiland small particles were filtered under vacuum with fast flow filterpaper lining a 10.5 cm Buchner funnel. Three filter papers were used tofilter about 1600 ml of oil each time. The filter papers were then setaside.

The particle cake left behind in the batch vessel was then resuspendedin isopropanol, small portions at a time (200 ml of isopropanol was usedto resuspend about 1/5 of the particles). The suspended particles werepoured into the same Buchner funnel under vacuum but lined with a newsheet of fast flow filter paper. The above two steps were repeated 5times until all of the particles had been transferred to the funnel. Theresidue of particles from the batch vessel and the small particles onthe three previous filter papers were also rinsed into the funnel usinga total of about 600 ml of isopropanol.

After the isopropanol coming out of the bottom of the funnel slowed to adrip, the particles were rinsed with 200 ml of isopropanol. This stepwas repeated twice, waiting for the isopropanol to slow to a drip beforerepeating. The total isopropanol used to wash the particles was about2.2 liters. The particles were then completely dried under vacuum.

Finally, in the fifth step of the process, the particles were sinteredat high temperature to remove residual organic impurities, eliminatemicropores, and provide strength by increasing intercolloidal bonding.The particles were transferred to a porcelain crucible large enough sothat the depth of particles was no more than 2.5 cm. Using a programmedtemperature oven (NEC), the particles were heated for 2 hours at 375° C.to burn organics, for 6 hours at 750° C. to burn off carbon andnitrogen, and for 3 hours at 900° C. to sinter the colloid bonds. A 40°C./minute temperature ramp was used to reach each temperature. Theparticles were characterized in the same way the SOM particles were todetermine particle size distribution, particle surface area, porevolume, apparent density, effective density, and morphology. Before use,the particles were acid and base treated to establish a consistentsurface chemistry, and classified with elutriation, as with theparticles generated by the SOM process.

C. Expanded Bed Methods

Fluidization and protein adsorption studies were carried out in a 2.5×15cm glass column (Kontes) with a hydraulic adjustable Teflon plunger andTeflon end fittings. The plunger was equipped with either polymeric fritor stainless steel screen (38 μm) on the end exposed to the particlebed. Both sintered stainless steel frit and 8 micron, twilled dutchweave, stainless steel screens (Small Parts, Inc.) were evaluated forinlet flow distributors. Inlet screens were attached to a modified 45°flow distribution system (FIG. 1) or a modified column end fitting whichhas a shallow 4° cone. The screen was secured to the end fitting witheither a flat neoprene binder ring and a thin O-ring (FIG. 1A) or with aTeflon sealing ring (FIG. 1B). The stainless steel screen was replacedwith a new screen after every third experiment. The fluidized bed wasconnected to a peristaltic pump (Buchler or Cole Parmer) using 0.8 mm idTeflon tubing and tube fittings (Chrom Tech). Either Tygon or Cflextubing was used in the peristaltic pump. A 316 stainless steel pressuregauge (ABM) was attached via a three-way connector between the pump andthe column inlet. Protein effluent absorbance was measured at 280_(nm)using a UV detector (Pharmacia UV-MII) and the signal recorded on achart recorder (Linseis L6012). Flow rates were measured with a glassgraduated cylinder and a stopwatch. Column height to diameter ratio was2.1-2.3 cm to 2.5 cm.

D. Determination of Flow Hydrodynamics

Residence time distribution ("RTD") studies were performed using atracer stimulus method, as described in N. M. Draeger et al., Trans.Int. Chem. Eng., 69, 45 (1991), and Wnukowski and A. Lindgren, Recoveryof Biological Products IV, Engineering Foundation Conference, Interlaken(1992), to assess the degree of dispersion in the liquid phase inrelation to column inlet geometry and fluidization velocity. NaNO₂ (0.5ml of 1 M) in 100 mM NaF, 50 mM MES buffer (pH adjusted to 5.5 with 5 MKOH) was injected into the flow system using a 0.8 mm id Teflonmultiport sample injection valve (Rheodyne model 5020, Chrom Tech, AppleValley, Minn.) placed between the fluid reservoir and the pump. The UVabsorbance of NO₂ ⁻ in the column effluent was measured at 254_(nm)using a flow cell detector with a total cell volume of 30 μl (UV-M IIMonitor, Pharmacia LKB Biotechnology, Piscataway, N.J.). Peak data wererecorded every 0.1 second using a Data Acquisition software (Rainin,Woburn, Mass.).

The relative degree of mixing in the liquid phase was measured for theentire system (tubing, fittings, bed, particles, detector) as well asthe system tubing alone with the column bypassed by connecting thecolumn inlet directly to the column outlet. System dispersion as afunction of column inlet flow geometry was measured without particles inthe column by resting the hydraulic plunger on top of the teflon ring(see FIG. 1B) with the screen in place. Two inlet flow geometries werestudied as a function of bed expansion: the 4° shallow cone and a 45°cone (FIG. 1).

The tracer, NaNO₂, was shown not to interact with the zirconia surfacein the presence of fluoride by comparing the retention time of repeatedinjections of 1 M NaNO₂ at 100 mM and 200 mM NaF. The retention time wasequal at both NaF concentrations. For bed expansion RTD measurements,the particle bed was first expanded to the appropriate height andallowed to stabilize for approximately 5 minutes before tracerinjection.

The zeroth moment, peak asymmetry, and peak area/maximum peak heightwere calculated for each tracer effluent peak. The zeroth moment wascalculated as: m_(o) =Q*Δt* ΣC_(i), where Q is the volumetric flow rate(ml/minute), Δt is the data acquisition time interval (minutes), andC_(i) is the UV monitor voltage output (μV). The summation limits were3% of the maximum peak height. Peak asymmetry was measured at 50 percentof maximum peak height and calculated as: (c-b)/(b-a), where b is thetime of tracer peak maximum, (c-b) is the time length of the peak tail,and (b-a) is the time of the length of the peak front.

E. Determination of Dynamic Binding Capacity

Bovine serum albumin (BSA) was used as a model protein in order todetermine protein adsorption during fluidization and to compare thecharacteristics of fluoride-modified zirconium oxide particles withprevious studies of BSA adsorption using fluidized, polymericion-exchange adsorbents. The surface of bare zirconia particles wasmodified by adsorption of fluoride from the mobile phase loading buffer(50 mM MES, 100 mM NaF, pH 5.5), which is described by J. A. Blackwellet al., J. Chromatogr., 549, 59 (1991). Adsorbed fluoride was strippedduring column washing with 0.25 M NaOH but the surface could beregenerated by washing the column with fluoride buffer. BSA was adsorbedonto the fluoride-modified zirconium oxide particles from a loadingconcentration, C_(o), of 4 mg/ml. BSA was eluted by a step increase inionic strength using an elution buffer consisting of 50 mM MES, 100 mMNaF and 750 mM Na₂ SO₄, pH 5.5, which is described by J. A. Blackwell etal., J. Chromatogr., 549, 59 (1991).

Each BSA breakthrough procedure consisted of first washing withapproximately 100 ml of 0.1-0.25 M NaOH, equilibration with loadingbuffer until the pH returned to within 0.2 pH units of pH 5.5, followedby BSA loading, washing with loading buffer, eluting adsorbed BSA withelution buffer, cleaning with 0.25 M NaOH, and rinsing withdouble-distilled H₂ O until the effluent pH was within 0.5 pH units ofthat of distilled H₂ O. Before each adsorption experiment, the particleswere gently agitated and allowed to settle for even distribution, thecolumn was checked for vertical orientation, and the plunger lowered toone centimeter above where the height of the expanded bed would be.

For packed bed experiments at constant linear velocity, the plunger waspositioned on top of the settled bed. All solutions were filteredthrough a 0.45 μm filter (Gelman brand filter). All solutions except theloading solution which included BSA were degassed by stirring undervacuum for several minutes. BSA solutions were either made fresh at thestart of each experiment or stored at 4° C. for at most several daysprior to use. Protein adsorption breakthrough profiles were generatedeither by protein loading while gradually increasing the flow rate andexpanding the bed to its final porosity over a period of 2-2.5 minutes(FIG. 9), or protein loading following prior establishment of a stablefully expanded bed.

Dynamic binding capacity (DBC) was calculated from BSA UV adsorptionbreakthrough curves recorded on a chart recorder and photographicallyenlarged to twice the original size (in order to more accuratelydetermine UV absorbance as a function of time) by multiplying the inletconcentration of the loading solution, C_(o), by the product of theaverage volumetric flow rate, Q, and the time since the start of loadingdivided by the settled bed volume. Settled bed volume was determinedfrom the observed bed height, H, and the column inner radius. The DBCwas also calculated using the graphical method of C. J. Geankoplis,Transport Processes and Unit Operations, p. 700, Prentice Hall, NewYork, (1993), by the product of the area above the breakthrough curve upto the breakthrough point and (C_(o))(Q)/(H). Dynamic binding capacitywas calculated at C/C_(o) =0.01 (1% of BSA feed concentration) and 0.05(5% of BSA feed concentration) in mg BSA loaded per ml of settled bed.Total bed capacity was calculated as the product of the total area abovethe curve with (C_(o))(Q)/(H).

F. Bed Cleaning

Particles were routinely cleaned of adsorbed protein using 0.1 or 0.25 MNaOH during bed expansion by flushing with at least 100 ml of base. Morevigorous cleaning of the particles was achieved by removing them fromthe column into a polypropylene flask with 250 ml 0.5 M NaOH withshaking at 300 rpm at 50° C. for 64 hours. Between adsorptionexperiments, the settled particle bed was saturated withdouble-distilled H₂ O or equilibration buffer.

G. Preparation of Dextran-Coated Zirconia Particles

The dextran (9300 MW) and the iodoacetic acid were obtained from SigmaChemical Co. (St Louis, Mo.). Boron trifluoride diethyl etherate and1,4-butanediol diglycidyl ether (BUDGE) were obtained from AldrichChemical Co. (Milwaukee, Wis.). All water was deionized and then passedthrough Barnstead ion Exchange and Organic Free cartridges followed by a0.45 μm filter. All water was also boiled for 15 minutes to removecarbon dioxide.

Ten grams of dextran was dissolved in 40 ml of water and then cooled to0-4° C. using an ice bath. A freshly made 12.5 M sodium hydroxidesolution (40 ml) was then added. The solution was then stirred at 4° C.for 30 minutes. Iodoacetic acid (10.5 g) was added gradually over 10minutes. After all of the iodoacetic acid had been added, the solutionwas stirred for 10 minutes at 4° C. for 10 minutes. The temperature wasthen increased to 60° C. and the solution was stirred for 30 minutes.The solution gradually became darker yellow as the reaction proceeded.The solution was then cooled in an ice bath. The pH was reduced to 9with concentrated HCl. Methanol was then added to the solution graduallywhile stirring to precipitate the carboxymethyl dextran (CMD). Thestirrer was turned off when the white precipitate stopped forming. Afterthe precipitate settled to the bottom, the supernatant was decanted. Theprecipitate was then redissolved in water and reprecipitated usingmethanol. The twice precipitated CMD had a slight yellow color, probablydue to residual iodine. The substitution of carboxylic groups wasdetermined by using the assay of Horikawa and Tanimura as disclosed inT. Horikawa et al., Anal. Lett., 15, 1629 (1982). Acetic acid was usedfor the calibration curves. The average percent substitution was 5.1%,or about 3 carboxymethyl groups per chain (57 glucose monomers in a 9300MW dextran chain).

The coating method is based. on that of X. Santarelli et al., J.Chromatogr., 443, 55, (1988). Carboxymethylated dextran (CMD) (0.1 g)(MW 9300, 5% substitution) was dissolved in 50 ml of 100 mM PIPES(piperazine-N,N'-bis[2-ethanesulfonic acid]), pH 6.5 to make a 2 g/lsolution of CMD. To 40 ml of this solution, 4 g of zirconia particleswere added. This suspension was sonicated under vacuum for five minutesand then capped. The bottle was then placed on a shaker bath for 2 days,with periodic manual shaking.

After the allotted time, the particles were allowed to settle and thesupernatant was decanted. Ethanol (40 ml) was then added and the slurrywas shaken for 10 minutes. The particles were allowed to settle for 30minutes and the ethanol was removed by suction. This procedure wasrepeated for 50:50 ethanol:chloroform (vol:vol) and chloroform. Theparticles were then allowed to air dry at room temperature.

The coated particles were placed in a 30 ml septum flask and 10 mL ofchloroform were added. The flask was capped and sonicated for 5 minutes.1,4-Butanediol diglycidyl ether (BUDGE, 17 μl) was added and the flaskwas sealed while nitrogen was blown over the top. In a separate septumflask, 7 mi of chloroform was added and sealed. Boron trifluorideetherate (0.5 ml) was then added by syringe. This solution (0.5 ml) wasthen added to the flask containing the coated particles. The particlesuspension was swirled and then allowed to sit for 30-40 minutes. Afterthis time, the solution was removed and the particles were rinsed withethanol.

H. Modification of Dextran Coating with a Dye

The protocol for carrying out the Cibacron-Blue coupling reaction isdescribed in Bohme et al., J. Chromat., 69, 209 (1972). The scale wasreduced to one-third. A quantity of 0.67 g of Cibacron-Blue wasdissolved in 20 ml of double-distilled water. Previously drieddextran-coated zirconia particles (3.2 g) were suspended in 120 ml ofdouble-distilled water. The particles were sonicated under vacuum for 15minutes to remove gases from the particle pores. The Cibacron-Bluesolution was then added to the particle suspension and stirred at 60° C.After 30 minutes, 15 g of NaCl was added and the mixture was stirred for3 hours at 75° C. This was then cooled, washed with copious amounts ofdouble-distilled water, and dried in a 100° C. vacuum oven.

I. Modification of Dextran Coating with a Thiophilic Ligand

Synthesis of thiophilic, affinity-phase coatings of porous zirconiaparticles involved reacting divinylsulphone (DVS) with dextran-coatedparticles followed by an end-capping reaction using, for example,mercaptoethanol (HSEtOH) or mercaptopyridine. Previously drieddextran-coated particles (2.8 g) were mixed with 5.6 ml of 0.5 M Na₂ CO₃in a 50 ml Erlenmeyer flask. The particles were sonicated under vacuumfor 15 minutes to remove gases from the particle pores. A solution of1.75 ml DVS and 1.75 ml isopropanol was made and combined with theparticle solution. The mixture was then shaken at 200 rpm to react theDVS with the dextran coating. After 23.75 hours, the thio-modifiedparticles were washed first with isopropanol followed bydouble-distilled water. Washing was done by swirling the particles incopious amounts of solvent, settling the particles, and decanting theliquid.

When clean, the particles were ready for the end-capping. Na₂ CO₃ 0.1 M,35 ml) was added to the particles to resusend them followed by 3.5 ml ofHSEtOH or mercaptopyridine. This mixture was then shaken at 250 rpm for6 hours to fully react the ends of the thiophilic coating. The particleswere again washed in double-distilled water and were then dried in a100° C. vacuum oven.

II. RESULTS

A. Characterization of Particles

The average diameter after elutriation of SOM particles, as determinedby particle screening, and the average hydraulic pore diameterdetermined by nitrogen porisimetry, varied slightly from batch to batch(Table I) depending upon the extent of removal of water during synthesis(i.e., how much water was present when the particles were collected andwashed). The weight percentage of particles as a function of size wasdetermined for an individual batch ("SOM A") and combined batches ("SOMA & B") by screening. Particles prepared by the SOM synthesis weregenerally spherical with some particles slightly flattened or indentedon one side (FIG. 3A). This variation in particle shape is most likelythe result of mixing conditions during particle formation. Particlesprepared by the FBOM synthesis were predominantly spherical with veryfew flattened or indented portions (FIG. 3B). Classification byelutriation eliminated adhering surface zirconia resulting in a smoothporous particle surface.

Determination of the effective particle density from direct apparentdensity measurements of dry SOM A particles (3.32±0.15 g/cm³) and fromnitrogen porisimetry measurements (3.30±0.10 g/ cm³) agreed well (TableI) with the determination of particle density by fluidization velocityusing the Richardson-Zaki relationship, as disclosed in J. F. Richardsonet al., Trans. Inst. Chem. Eng., 32, 35 (1954), despite the fact thatthis relationship is theoretically for particles of 100 μm or larger.

                  TABLE I                                                         ______________________________________                                        Characterization of SOM Particles                                                          SOM A      SOM A & B                                             ______________________________________                                        Mean Diameter  54 μm     43 μm                                          (determined by screening)                                                     Apparent Density                                                              (gavitimetric)                                                                Apparent Density                                                                             1.803 ± 0.017 g/cm.sup.3                                                                1.696 ± 0.014 g/cm.sup.3                       Void Fraction  0.51 ± 0.03                                                                             0.54 ± 0.03                                    Effective Density                                                                            3.32 ± 0.15 g/cm.sup.3                                                                  3.18 ± 0.15 g/cm.sup.3                         Nitrogen Porisimetry                                                          Surface Area   21.7 ± 0.2 m.sup.2 g                                                                    21.5 ± 0.2 m.sup.2 g                           Void Volume    0.183 ± 0.002 cm.sup.3 /g                                                               0.196 ± 0.002 cm.sup.3 /g                      Hydraulic Pore Diameter                                                                      338 Å    366 Å                                         (4V/A)                                                                        BET Adsorption Peak                                                                          516 Å    580 Å                                         BET Desorption Peaks                                                                         292 Å, 310 Å                                                                       312 Å, 363 Å                              Void Fraction  0.51 ± 0.02                                                                             0.53 ± 0.02                                    Effective Density                                                                            3.30 ± 0.10 g/cm.sup.3                                                                  3.22 ± 0.09 g/cm.sup.3                         ______________________________________                                    

The results of two FBOM batches are summarized in Tables II, III, andIV. Table II, "Comparison of Yields," shows the percent and total yieldsfor the FBOM batches and compares them to the SOM technology. Table III,"Comparison of Analyses," shows the measured and observedcharacteristics of the FBOM batches and compares them with the SOMtechnology. Table IV, "Particle Comparison Table," compares the measuredcharacteristics and extent of drying of FBOM and SOM particles. It canbe seen from the "Yield" table that much higher yields are obtained withthe FBOM process than with the SOM process. A final yield of about 31 to41% of the zirconia available in the original colloid is obtained in thefinal FBOM particles. The yield for SOM A was only 6.4%.

                  TABLE II                                                        ______________________________________                                        Comparison of Yields                                                                        FBOM A      FBOM B      SOM A                                   ______________________________________                                        Mass of zirconia in                                                                           540 g         540 g     517 g                                 starting sol                                                                  Mass of zirconia colloid in                                                                   356 g         356 g     80 g                                  concentrated sol after                                                        centrifugation,                                                               resuspension                                                                  Yield of zirconia colloid in                                                                  66%           66%       15%                                   concentrated sol after                                                        centrifugation,                                                               resuspension                                                                  Mass of particles after                                                                       330.98 g      331.16 g  60 g                                  synthesis, sintering                                                          Yield of particles from                                                                       92.9%         93.0%     75%                                   concentrated sol                                                              Yield of screened 38-74                                                                       75.1%         73.0%     81%                                   μm particles after sintering                                               Expected yield of                                                                             70-90%        70-90%    70%                                   base/acid washing and                                                         elutriation step                                                              Total Yield     32-41%        31-40%    6.4%                                  Expected total mass                                                                           173-224 g     167-218 g 33 g                                  available in final form                                                       Number of 3:1 H:D 2.5                                                                         2.8 to 3.7    2.8 to 3.6                                                                              0.55                                  cm diameter columns                                                                           columns       columns   columns                               (60 g/column)                                                                 ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Comparison of Analyses                                                        Analysis FBOM A      FBOM B      SOM B                                        ______________________________________                                        SEM      Spherical   Spherical   Mixture of                                   Morphology                       "chopped" &                                                                   spherical                                    Colloid size                                                                           1300     Å  1220   Å                                                                              1460   Å                             (Coulter)                                                                     Colloid size                                                                           260-860  Å  260-860                                                                              Å                                                                              >860   Å                             (Stoke's eq.)                                                                          avg. 700 Å  avg. 700                                                                             Å                                                                              avg. 1200                                                                            Å                             Mean particle                                                                          50       μm  48     μm                                                                              54     μm                             diameter after                                                                sintering, by                                                                 screening                                                                     Nitrogen 446      Å  433    Å                                                                              516    Å                             adsorption peak                                                                        very narrow very narrow                                              Nitrogen 263      Å  265    Å                                                                              310    Å                             desorption peak                                                                        very narrow very narrow                                              BET surface                                                                            28.9     m.sup.2 /g                                                                           27.9   m.sup.2 g                                                                          21.7   m.sup.2 g                         area                                                                          Single pt. vol                                                                         0.230    cm.sup.3 /g                                                                          0.224  cm.sup.3 /g                                                                        0.183  cm.sup.3 /g                       of pores                                                                      Hyd dia of                                                                             320      Å  321    Å                                                                              369    Å                             pores (4V/A)                                                                  Effective                                                                              3.03     g/cm.sup.3                                                                           3.07   g/cm.sup.3                                                                         3.30   g/cm.sup.3                        water-filled                                                                  density                                                                       (N.sub.2 data)                                                                Apparent 1.63     g/cm.sup.3                                                                           1.63   g/cm.sup.3                                                                         1.80   g/cm.sup.3                        density                                                                       Effective                                                                              3.10     g/cm.sup.3                                                                           3.10   g/cm.sup.3                                                                         3.32   g/cm.sup.3                        water-filled                                                                  density                                                                       (App. density)                                                                Particle void                                                                          0.57            0.56        0.51                                     fraction                                                                      (N.sub.2 data)                                                                Particle void                                                                          0.55            0.55        0.51                                     fraction                                                                      (App. density)                                                                ______________________________________                                    

                                      TABLE IV                                    __________________________________________________________________________    Particle Comparison Table                                                              SOM A SOM A & B                                                                           SOM B FBOM A                                                                              FBOM B                                                                              FBOM C                                                                              FBOM D                                                                              FBOM                                                                                FBOM                 __________________________________________________________________________                                                             F                    Colloid size                                                                           1460                                                                             Å                                                                            1460                                                                             Å                                                                            1460                                                                             Å                                                                            1300                                                                             Å                                                                            1220                                                                             Å                                                                            1460                                                                             Å                                                                            1250                                                                             Å                                                                            1250                                                                             Å                                                                            1460                                                                             Å             Surface Area                                                                           21.7                                                                             m.sup.2 /g                                                                       21.5                                                                             m.sup.2 g                                                                        22.3                                                                             m.sup.2 /g                                                                       28.9                                                                             m.sup.2 /g                                                                       27.9                                                                             m.sup.2 /g                                                                       24.4                                                                             m.sup.2 /g                                                                       30.5                                                                             m.sup.2 /g                                                                       30.9                                                                             m.sup.2 /g                                                                       26.6                                                                             m.sup.2 /g        Void Volume                                                                            0.183                                                                            cm.sup.3 /g                                                                      0.196                                                                            cm.sup.3 /g                                                                      0.204                                                                            cm.sup.3 /g                                                                      0.230                                                                            cm.sup.3 /g                                                                      0.224                                                                            cm.sup.3 /g                                                                      0.221                                                                            cm.sup.3 /g                                                                      0.242                                                                            cm.sup.3 /g                                                                      0.218                                                                            cm.sup.3 /g                                                                      0.210                                                                            cm.sup.3 /g       Hyd. dia. of pores                                                                     338                                                                              Å                                                                            366                                                                              Å                                                                            366                                                                              Å                                                                            320                                                                              Å                                                                            321                                                                              Å                                                                            363                                                                              Å                                                                            317                                                                              Å                                                                            282                                                                              Å                                                                            317                                                                              Å             (4 V/A)                                                                       BET Nit Ads Peak                                                                       516                                                                              Å                                                                            580                                                                              Å                                                                            879                                                                              Å                                                                            446                                                                              Å                                                                            433                                                                              Å                                                                            901                                                                              Å                                                                            448                                                                              Å                                                                            411                                                                              Å                                                                            515                                                                              Å             BET Nit Des Peak                                                                       310                                                                              Å                                                                            312                                                                              Å                                                                            395                                                                              Å                                                                            263                                                                              Å                                                                            265                                                                              Å                                                                            396                                                                              Å                                                                            288                                                                              Å                                                                            264                                                                              Å                                                                            332                                                                              Å                            363                                                                              Å                                                       Void Fraction                                                                          0.51  0.53  0.54  0.57  0.56  0.56  0.58  0.55  0.54                 Eff. Density                                                                           3.30                                                                             g/cm.sup.3                                                                       3.22                                                                             g/cm.sup.3                                                                       3.19                                                                             g/cm.sup.3                                                                       3.03                                                                             g/cm.sup.3                                                                       3.07                                                                             g/cm.sup.3                                                                       3.08                                                                             g/cm.sup.3                                                                       2.98                                                                             g/cm.sup.3                                                                       3.10                                                                             g/cm.sup.3                                                                       3.14                                                                             g/cm.sup.3        __________________________________________________________________________

B. Characterization of the Hydrodynamic Behavior of Expanded Beds

The flow behavior of porous SOM particles was characterized in water andin buffer containing BSA. The relationship between bed porosity, ε, andsuperficial liquid velocity, u, were found to fit well to theRichardson-Zaki relationship, u=u_(t) ε^(n), as discussed in J. F.Richardson et al., Trans. Inst. Chem. Eng., 32, 35 (1954), with n≈5.55indicating laminar flow. See, Y. S. Chong et al., Powder Technology, 23,55 (1979); J. F. Richardson et al., Trans. Chem. E., 39, 348 (1961); andL. G. Gibilaro et al., Chem. Eng Sci., 40, 1817 (1985). The terminalvelocity, u_(t), was determined to be 2.7-3.1 mm/second for SOMparticles depending upon the particle size distribution of each batch.This observed terminal settling velocity agreed well with the valuepredicted from the Stokes' equation, on substitution of appropriatevalues for the physical characteristics for each size of zirconium oxideparticles. Protein adsorption studies were carried out with SOMparticles whose linear flow velocities were in the range of 100-220cm/hour (FIG. 4). Settled packed bed (1× bed expansion) studies werecarried out at a linear velocity of approximately 100 cm/hour.

The residence time distributions for the tubing system (withoutparticles in the bed) and for the complete system were determined as afunction of bed expansion and compared for the standard 4° and conical45° flow adapters. With SOM particles in the column (packed bed) bimodalpeaks were observed using the 4° flow distributor. For this reason, theshallow cone was modified as shown in FIG. 1B which eliminated peaksplitting in the tracer pulse signal (FIGS. 5A and 5B).

While the residence time distributions for either flow distributionsystem at constant flow rate were sharp for the tubing alone and forpacked SOM particles, as expected, significant peak broadening wasobserved as a function of bed expansion for both flow distributors (seeFIG. 5B for the 45° inlet flow adapter). Peak broadening was quantifiedby the ratio of peak area to peak height. Contributing to this observedbroadening is the 1 cm dead volume between the expanded bed surface andthe column outlet. Peak zeroth moment did not differ significantlybetween the two flow distribution systems with increasing flow rateeither in the absence of particles or for the complete system. In thepresence of particles, peak asymmetry increased as a function of flowrate and was greater with the 4° flow distributer. The modified flowdistributer shown in FIG. 1A resulted in observable jetting in thecolumn. For these reasons, BSA adsorption studies were carried out usingthe 45° flow distributor shown in FIG. 1B.

C. Determination of BSA Adsorption and Bed Expansion

BSA desorption was compared between a fixed (settled) and expanded bedof SOM particles (FIG. 6). Adsorption and elution profiles for expandedbeds were comparable to the packed bed (FIG. 7). Even though significantpeak broadening occurred during adsorption at 2×, 2.5×, and 3× bedexpansion, BSA elution by altering the ionic strength at constant flow,and without reversal of flow, was rapid and resulted in a reproducible,sharp elution peak (FIG. 7). This elution method is in contrast tosettling of the bed and elution by reversing flow direction which isused for less dense larger expanded bed adsorbents.

DBC varied at constant flow velocity for the packed bed and bedexpansion up to 3-fold for SOM particles (FIG. 8) as superficialvelocity was increased from 109 to 220 cm/hour (Table V). The twomethods used to determine DBC were in good agreement (FIG. 8). Anindication of the effect of dispersion in the expanded bed as a functionof bed expansion was obtained from the slope of the BSA breakthroughcurves at C/C_(o) =0.5, as described in N. M. Draeger et al., Trans.Int. Chem. Eng., 69, 45 (1991), and P. Wnukowski et al., Recovery ofBiological Products IV, Engineering Foundation Conference, Interlaken(1992).

The method of BSA loading, whether at full expansion prior to loading orwhile gradually expanding the bed to full expansion during loading, didnot significantly affect the DBC. The time to fully expand the bed whenswitching to BSA loading buffer from equilibration buffer (1.9-2.7minutes) was approximately one third of the time to 1% BSA breakthroughdepending on fluidization velocity (FIG. 9).

The variation in DBC ranged from 54 to 66 mg BSA/ml settled beddepending upon fluidization velocity (Table V) when the bed was expandedto greater than 3-fold. This increase in DBC was unexpected given thatmass transfer limitations predict decreasing protein adsorption as bedporosity increases. Table V indicates that comparable BSA bindingcapacities to much deeper beds of larger DEAE particles with largesettled bed height to diameter ratios can be achieved at very highliquid velocities (>100 cm/hour) even with shallow beds of less than 2.5cm bed height using 43 μm to 54 μm zirconium particles. These zirconiaparticles allow fluidization at a velocity high enough to reach optimumexpansion to minimize dispersion and to allow separation of theadsorbent from entrained solids but with a reduced characteristicdiffusion length within the adsorbent compared to currently availabletechnology.

                                      TABLE V                                     __________________________________________________________________________    Comparison of BSA Adsorption Characteristics for Porous Zirconium Oxide       Particles                                                                                               DYNAMIC BINDING CAPACITY                                           FLOW RATE BED H:D                                                                        (mg BSA/ml Settled Bed)                             PARTICLES                                                                           BED EXPANSION                                                                          (cm/hr)                                                                             (cm) 1% Breakthrough (n)                                                                     5% Breakthrough (n)                       __________________________________________________________________________    SOM-A 1 ×                                                                              109   2:2.5                                                                              41 ± 6 (1)                                                                           44 ± 6 (1)                                   2.5 ×                                                                            164        30 ± 3 (2)                                                                           40 ± 4 (2)                                   3 ×                                                                              215        41 ± 6 (2)                                                                           47 ± 1 (2)                             SOM A & B                                                                           1 ×                                                                              109   2:3:2.5                                                                            26 ± 2 (3)                                                                           34 ± 2 (3)                                   2 ×                                                                              110        29 ± 1 (3)                                                                           37 ± 1 (3)                                   2.5 ×                                                                            158        26 ± 3 (3)                                                                           35 ± 3 (3)                                   3 ×                                                                              200        28 ± 2 (3)                                                                           35 ± 2 (3)                                   3.4 ×                                                                            220        29 ± 3 (1)                                                                           33 ± 3 (1)                             __________________________________________________________________________     Error is ± one standard deviation, n = number of individual                determinations                                                           

D. Reproducibility of BSA Binding Capacity

Protein adsorption studies using the same column of SOM particles overan eight month period of time revealed several behaviors that resultedin dramatic decreases in BSA DBC. Bed dispersion and DBC were primarilyaffected by the column inlet flow geometry, since the ratio of columndiameter to particle diameter of 500 rendered wall effectsinsignificant. Similar DBC was obtained using the stainless steel fritor the screen (FIG. 10A), however, the location of the retaining ring onthe initial screen design (FIG. 1A) resulted in generation of smallfluid jets at the base of the bed which reduced the DBC. Jets wereeliminated by altering this design to that shown in FIG. 1B. The fritwas more prone to clogging with protein during repetitive adsorption anddesorption experiments and could not be adequately cleaned with 0.1 MNaOH. Frit clogging dramatically reduced the DBC from 30±2 mg/ml settledbed volume to 12±5 mg/ml settled bed volume. Insufficient cleaning with0.1 M NaOH resulted in particle clumping which also reduced DBC morethan three fold (FIG. 10B). Clumped particles could be cleaned bystripping with 0.25-0.5 M NaOH with shaking at 50° C. to restore BSAbinding capacity to the original level.

E. Performance of Zirconium Oxide Particles After Repeated Cleaning

Repeated cycles of BSA binding, elution and column stripping with NaOHdid not result in significant particle fracture, generation of fines(detected as cloudiness at the expanded bed interface), or loss ofdynamic binding capacity over a nine month period of SOM particleevaluation. During this time period, the particles were exposed to over500 column volumes of NaOH both at ambient and at elevated temperatures.Routine storage of the column was feasible at low pH (pH 5.5) withoutthe need for addition of sodium azide. No microbial growth was evidentduring this time period using low ionic strength fluoride buffer inspite of column storage at ambient temperature. The dynamic bindingcapacity at a 2-fold bed expansion observed on the initial breakthroughdeterminations was 30±2 mg/ml settled bed volume. The DBC at the end ofeight months of column use with repeated cleaning with NaOH was 32±4mg/ml settled bed volume. Particle size and size distribution did notappear to be altered as a result of months of repeatedadsorption/desorption and base washing cycles.

F. Protein Adsorption at Elevated Temperatures

Repeated cycles of BSA binding at elevated temperatures indicate thatDBC increases as temperature increases (FIG. 11). This data was obtainedusing a column similar to FIG. 1 with a hot water circulation jacketsystem. Inlet fluids were preheated to the indicated temperature.

By elevating the loading temperature from 25° C. to approximately 45°C., DBC increases by one-third. The column was rapidly eluted, cleanedwith base, and reequilibrated at each indicated temperature. In eachcase, DBC increased with increasing temperatures, and column cleaningbecame more efficient using strong base at elevated temperatures.

The complete disclosures of all patents and publications cited hereinare incorporated by reference as if individually incorporated byreference. While this invention has been described in connection withspecific embodiments, it should be understood that it is capable offurther modification. The claims are intended to cover those variationswhich one skilled in the art would recognize as the chemical equivalentof what has been described herein. Thus, various omissions,modifications, and changes to the principles described herein can bemade by one skilled in the art without departing from the true scope andspirit of the invention, which is indicated by the following claims.

What is claimed is:
 1. An expanded bed system comprisingsurface-modified zirconium oxide particles having a capacity factorgreater than about 10; said particles comprising a core zirconium oxideparticle having a particle size of about 30-400 μm and a specificgravity of about 2.5-3.5 g/cm³.
 2. The expanded bed system of claim 1wherein the specific gravity of the core zirconium oxide particle isabout 3.0-3.5 g/cm³.
 3. The expanded bed system of claim 1 wherein theparticle size of the core zirconium oxide particle is about 50-200 μm.4. The expanded bed system of claim 1 wherein the surface-modifiedzirconium oxide particles have a capacity factor greater than about 50.5. The expanded bed system of claim 1, wherein the surface modifiedzirconium oxide particles have pores.
 6. The expanded bed system ofclaim 5, wherein the pores have a pore size of about 200-1500 Å.
 7. Theexpanded bed system of claim 5, wherein at least about 70% of the poreshave a pore size within a range of ±50% of a pore size average of about200-1500 Å.
 8. The expanded bed system of claim 1, wherein thesurface-modified zirconium oxide particles comprise a coating of amaterial selected from the group consisting of a hydrophilic polymer, acarbohydrate polymer having a covalently bound affinity ligand, and aLewis base.
 9. The expanded bed system of claim 8, wherein thehydrophilic polymer comprises polyethyleneimnine.
 10. An expanded bedcomprising surface-modified zirconium oxide particles having a capacityfactor greater than about 10, wherein the surface-modified zirconiumoxide particles comprise a core zirconium oxide particle having aparticle size of about 30-400 μm and a specific gravity of about 3.0-3.5g/cm³, wherein the expanded bed has a height to diameter ratio less thanabout 1:1.
 11. An expanded bed comprising surface-modified zirconiumoxide particles having a capacity factor greater than about 10, whereinthe surface-modified zirconium oxide particles comprise a core zirconiumoxide particle having a particle size of about 30-400 μm, a specificgravity of about 2.5-3.5 g/cm³, and an ion-exchange phase comprisingfluoride ions.
 12. An expanded bed comprising surface-modified zirconiumoxide particles having a capacity factor greater than about 10, whereinthe surface-modified zirconium oxide particles comprise a core zirconiumoxide particle having a particle size of about 30-400 μm, a specificgravity of about 2.5-3.5 g/cm³, and an affinity phase comprising dextranhaving covalently bound nonprotein affinity ligands.
 13. An expanded bedcomprising surface-modified zirconium oxide particles having a capacityfactor greater than about 10, wherein the surface-modified zirconiumoxide particles comprise a core zirconium oxide particle having aparticle size of about 30-400 μm, a specific gravity of about 2.5-3.5g/cm³, and an affinity phase comprising dextran having covalently boundtriazine dyes and thiophilic ligands.